The invention relates to a method of preparing a composite material comprising a silica network and, interwoven therewith, chains of a polyhydroxy compound, a mixture comprising an alkenyl-orthosilicate monomer and a photoinitiator being converted into said silica network by hydrolysis and polycondensation and into said polyhydroxy compound by radical polymerization.
The invention also relates to a liquid crystal display device comprising a transparent substrate having a colour-filter pattern which is covered by a top coat containing polyacrylate.
The invention further relates to a method of manufacturing such a device.
Such inorganic-organic copolymers are referred to as ORMOCERs.TM. (ORGanically MOdified CERamics), CIOMATS (Composite Inorganic Organic MATerials) or CERAMERs (CERAmic polyMERs)in literature and are used, for example, in optical components and as protective coatings for electronic and optical devices and as waveguides in micro-optical systems. In general, said materials are optically transparent in the visible wavelength range and adhere well to various substrates, such as ceramics, glass, metals and polymers.
The silica network forms a glass consisting of a three-dimensional structure of covalently bonded silicon oxides. The polymer chains occupy the empty spaces within the silica network and are interwoven therewith, while forming a composite material consisting of an inorganic matrix of silicon oxide and organic polymer chains. The organic polymer gives the composite material a high tensile strength, a high modulus of elasticity and a high impact strength, and the three-dimensional inorganic matrix gives the material a high hardness, a high scratch resistance and a high compressive strength. Such a composite material can be advantageously obtained by applying a sol-gel process because of the mild reaction temperatures used in said process. The sol-gel process is based on the homogeneous hydrolysis and condensation of metal alkoxides (metal M is, for example, Si, Ti, Zr, Ti, Ge, Al) in the presence of water to form cross-linked, swollen metal-oxide networks. A frequently used silicon alkoxide is tetraethyl orthosilicate: Si(OC.sub.2 H.sub.5).sub.4 (TEOS). When TEOS is brought into contact with water, the following reactions take place: EQU Hydrolysis: .tbd.Si--OC.sub.2 H.sub.5 +H.sub.2 O.fwdarw..tbd.Si--OH+C.sub.2 H.sub.5 OH (1) EQU Condensation: .tbd.Si--OH+HO--Si.tbd..fwdarw..tbd.Si--O--Si.tbd.+H.sub.2 O(2) EQU Net reaction: Si(OC.sub.2 H.sub.5).sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +4C.sub.2 H.sub.5 OH (3)
In accordance with reaction (2), a network of silica is formed (polycondensation). The reaction product is dried in order to remove the excess water and the liberated alcohol (ethanol in the above example). The mild reaction temperatures enable an organic oligomer or polymer to be dissolved in the sol-gel solution. After hydrolysis and condensation, the polymer is homogeneously embedded in the silica network under favourable conditions. However, few polymers can be dissolved in the sol-gel solution. Besides, extraordinary shrinkage (approximately 70% ) occurs upon drying the swollen network as a result of the evaporation of the liberated alcohol. Therefore, coatings of silica can only be made in very small thicknesses (&lt;0.1 .mu.m); at greater thicknesses said coatings crack.
In an article by B. M. Novak et at. in Macromolecules, 24, pp. 5481-5483 (1991) it is proposed to use silanes with polymerizable groups, such as tetraalkenyl orthosilicates, in such a method in place of silanes with methoxy or ethoxy groups commonly used in sol-gel processes. Silane tetra(hydroxyethyl acrylate ester) (STEA) (official name: tetrakis(2-hydroxy ethoxy)silane, tetra acrylate ester) is mentioned as an example of such a compound. The hydrolysis and condensation of this compound liberate a polymerizable unsaturated alcohol, namely 2-hydroxyethyl acrylate. By UV-exposure in the presence of a photoinitiator, but preferably thermally in the presence of a suitable catalyst mixture of ammonium persulphate and N,N,N', N'-tetramethylethylene diamine and by using a stoichiometric quantity of water, a silica network is formed at room temperature by hydrolysis and condensation of the silicate ester and, simultaneously, a polyacrylate is formed by radical polymerization of the acrylate groups. In this manner, a so-called interpenetrating network is formed. Since the liberated alcohol (2-hydroxyethyl acrylate) polymerizes to polyhydroxyethyl acrylate, drying of the gel formed is, in principle, unnecessary and hardly any shrinkage occurs during the polymerization step. According to said article, it is important that the inorganic silica network and the organic polymer are formed simultaneously. It is essential that the reaction rates of both polymerization processes are accurately matched. If the formation of the silica network takes place at a higher rate than the formation of the organic polymer, an opaque, brittle glass is formed which is also subject to shrinkage as a result of the evaporation of unreacted monomer. Dominant organic polymerization rates cause precipitation of said polymer, which results in phase separation. In either case, a turbid, highly scattering product is obtained which is unsuitable for optical applications. According to said article, it is tried to make both polymerization reactions proceed at similar rates by adding the appropriate quantity of ammonium persulphate to the reaction mixture. A disadvantage of this known method is that similar polymerization-reaction rates are difficult to achieve, so that the abovementioned adverse effects can occur.